Parameters

global match; find all matches rather than stopping after the first match

i

ignore case

m

multiline; treat beginning and end characters (^ and $) as working over multiple lines (i.e., match the beginning or end of each line (delimited by \n or \r), not only the very beginning or end of the whole input string)

u

unicode; treat pattern as a sequence of unicode code points

y

sticky; matches only from the index indicated by the lastIndex property of this regular expression in the target string (and does not attempt to match from any later indexes).

Description

There are 2 ways to create a RegExp object: a literal notation and a constructor. To indicate strings, the parameters to the literal notation do not use quotation marks while the parameters to the constructor function do use quotation marks. So the following expressions create the same regular expression:

/ab+c/i;
new RegExp('ab+c', 'i');
new RegExp(/ab+c/, 'i');

The literal notation provides compilation of the regular expression when the expression is evaluated. Use literal notation when the regular expression will remain constant. For example, if you use literal notation to construct a regular expression used in a loop, the regular expression won't be recompiled on each iteration.

The constructor of the regular expression object, for example, new RegExp('ab+c'), provides runtime compilation of the regular expression. Use the constructor function when you know the regular expression pattern will be changing, or you don't know the pattern and are getting it from another source, such as user input.

Starting with ECMAScript 6, new RegExp(/ab+c/, 'i') no longer throws a TypeError ("can't supply flags when constructing one RegExp from another") when the first argument is a RegExp and the second flags argument is present. A new RegExp from the arguments is created instead.

When using the constructor function, the normal string escape rules (preceding special characters with \ when included in a string) are necessary. For example, the following are equivalent:

Special characters meaning in regular expressions

(The dot, the decimal point) matches any single character except line terminators: \n, \r, \u2028 or \u2029.

Inside a character set, the dot loses its special meaning and matches a literal dot.

Note that the m multiline flag doesn't change the dot behavior. So to match a pattern across multiple lines, the character set [^] can be used (if you don't mean an old version of IE, of course), it will match any character including newlines.

For example, /.y/ matches "my" and "ay", but not "yes", in "yes make my day".

\d

Matches any digit (Arabic numeral). Equivalent to [0-9].

For example, /\d/ or /[0-9]/ matches "2" in "B2 is the suite number".

\D

Matches any character that is not a digit (Arabic numeral). Equivalent to [^0-9].

For example, /\D/ or /[^0-9]/ matches "B" in "B2 is the suite number".

\w

Matches any alphanumeric character from the basic Latin alphabet, including the underscore. Equivalent to [A-Za-z0-9_].

For example, /\w/ matches "a" in "apple", "5" in "$5.28", and "3" in "3D".

\W

Matches any character that is not a word character from the basic Latin alphabet. Equivalent to [^A-Za-z0-9_].

For example, /\W/ or /[^A-Za-z0-9_]/ matches "%" in "50%".

\s

Matches a single white space character, including space, tab, form feed, line feed and other Unicode spaces. Equivalent to [ \f\n\r\t\v\u00a0\u1680\u2000-\u200a\u2028\u2029\u202f\u205f\u3000\ufeff].

For example, /\s\w*/ matches " bar" in "foo bar".

\S

Matches a single character other than white space. Equivalent to [^ \f\n\r\t\v\u00a0\u1680\u2000-\u200a\u2028\u2029\u202f\u205f\u3000\ufeff].

For example, /\S\w*/ matches "foo" in "foo bar".

\t

Matches a horizontal tab.

\r

Matches a carriage return.

\n

Matches a linefeed.

\v

Matches a vertical tab.

\f

Matches a form-feed.

[\b]

Matches a backspace. (Not to be confused with \b)

\0

Matches a NUL character. Do not follow this with another digit.

\cX

Where X is a letter from A - Z. Matches a control character in a string.

(only when u flag is set) Matches the character with the Unicode value U+hhhh or U+hhhhh (hexadecimal digits).

\

For characters that are usually treated literally, indicates that the next character is special and not to be interpreted literally.

For example, /b/ matches the character "b". By placing a backslash in front of "b", that is by using /\b/, the character becomes special to mean match a word boundary.

or

For characters that are usually treated specially, indicates that the next character is not special and should be interpreted literally.

For example, "*" is a special character that means 0 or more occurrences of the preceding character should be matched; for example, /a*/ means match 0 or more "a"s. To match * literally, precede it with a backslash; for example, /a\*/ matches "a*".

Character Sets

Character

Meaning

[xyz]
[a-c]

A character set. Matches any one of the enclosed characters. You can specify a range of characters by using a hyphen, but if the hyphen appears as the first or last character enclosed in the square brackets it is taken as a literal hyphen to be included in the character set as a normal character. It is also possible to include a character class in a character set.

For example, [abcd] is the same as [a-d]. They match the "b" in "brisket" and the "c" in "chop".

For example, [abcd-] and [-abcd] match the "b" in "brisket", the "c" in "chop" and the "-" (hyphen) in "non-profit".

For example, [\w-] is the same as [A-Za-z0-9_-]. They match the "b" in "brisket", the "c" in "chop" and the "n" in "non-profit".

[^xyz]
[^a-c]

A negated or complemented character set. That is, it matches anything that is not enclosed in the brackets. You can specify a range of characters by using a hyphen, but if the hyphen appears as the first or last character enclosed in the square brackets it is taken as a literal hyphen to be included in the character set as a normal character.

For example, [^abc] is the same as [^a-c]. They initially match "o" in "bacon" and "h" in "chop".

Alternation

Character

Meaning

x|y

Matches either x or y.

For example, /green|red/ matches "green" in "green apple" and "red" in "red apple".

Boundaries

Character

Meaning

^

Matches beginning of input. If the multiline flag is set to true, also matches immediately after a line break character.

For example, /^A/ does not match the "A" in "an A", but does match the first "A" in "An A".

$

Matches end of input. If the multiline flag is set to true, also matches immediately before a line break character.

For example, /t$/ does not match the "t" in "eater", but does match it in "eat".

\b

Matches a word boundary. This is the position where a word character is not followed or preceded by another word-character, such as between a letter and a space. Note that a matched word boundary is not included in the match. In other words, the length of a matched word boundary is zero.

Examples:/\bm/ matches the 'm' in "moon" ;/oo\b/ does not match the 'oo' in "moon", because 'oo' is followed by 'n' which is a word character;/oon\b/ matches the 'oon' in "moon", because 'oon' is the end of the string, thus not followed by a word character;/\w\b\w/ will never match anything, because a word character can never be followed by both a non-word and a word character.

\B

Matches a non-word boundary. This is a position where the previous and next character are of the same type: Either both must be words, or both must be non-words. Such as between two letters or between two spaces. The beginning and end of a string are considered non-words. Same as the matched word boundary, the matched non-word bondary is also not included in the match.

The capturing groups are numbered according to the order of left parentheses of capturing groups, starting from 1. The matched substring can be recalled from the resulting array's elements [1], ..., [n] or from the predefined RegExp object's properties $1, ..., $9.

Capturing groups have a performance penalty. If you don't need the matched substring to be recalled, prefer non-capturing parentheses (see below).

\n

Where n is a positive integer. A back reference to the last substring matching the n parenthetical in the regular expression (counting left parentheses).

Matches x but does not remember the match. These are called non-capturing groups. The matched substring can not be recalled from the resulting array's elements [1], ..., [n] or from the predefined RegExp object's properties $1, ..., $9.

For example, /a+/ matches the "a" in "candy" and all the "a"'s in "caaaaaaandy".

x?

Matches the preceding item x 0 or 1 time.

For example, /e?le?/ matches the "el" in "angel" and the "le" in "angle."

If used immediately after any of the quantifiers *, +, ?, or {}, makes the quantifier non-greedy (matching the minimum number of times), as opposed to the default, which is greedy (matching the maximum number of times).

x{n}

Where n is a positive integer. Matches exactly n occurrences of the preceding item x.

For example, /a{2}/ doesn't match the "a" in "candy", but it matches all of the "a"'s in "caandy", and the first two "a"'s in "caaandy".

x{n,}

Where n is a positive integer. Matches at least n occurrences of the preceding item x.

For example, /a{2,}/ doesn't match the "a" in "candy", but matches all of the a's in "caandy" and in "caaaaaaandy".

x{n,m}

Where n and m are positive integers. Matches at least n and at most m occurrences of the preceding item x.

For example, /a{1,3}/ matches nothing in "cndy", the "a" in "candy", the two "a"'s in "caandy", and the first three "a"'s in "caaaaaaandy". Notice that when matching "caaaaaaandy", the match is "aaa", even though the original string had more "a"'s in it.

x*?x+?x??x{n}?x{n,}?x{n,m}?

Matches the preceding item x like *, +, ?, and {...} from above, however the match is the smallest possible match.

Quantifiers without ? are said to be greedy. Those with ? are called "non-greedy".

Assertions

Character

Meaning

x(?=y)

Matches x only if x is followed by y.

For example, /Jack(?=Sprat)/ matches "Jack" only if it is followed by "Sprat"./Jack(?=Sprat|Frost)/ matches "Jack" only if it is followed by "Sprat" or "Frost". However, neither "Sprat" nor "Frost" is part of the match results.

x(?!y)

Matches x only if x is not followed by y.

For example, /\d+(?!\.)/ matches a number only if it is not followed by a decimal point./\d+(?!\.)/.exec('3.141') matches "141" but not "3.141".

RegExp prototype objects and instances

Properties

Note that several of the RegExp properties have both long and short (Perl-like) names. Both names always refer to the same value. Perl is the programming language from which JavaScript modeled its regular expressions.

Examples

Using a regular expression to change data format

The following script uses the replace() method of the String instance to match a name in the format first last and output it in the format last, first. In the replacement text, the script uses $1 and $2 to indicate the results of the corresponding matching parentheses in the regular expression pattern.

Regular expression and Unicode characters

As mentioned above, \w or \W only matches ASCII based characters; for example, "a" to "z", "A" to "Z", "0" to "9" and "_". To match characters from other languages such as Cyrillic or Hebrew, use \uhhhh, where "hhhh" is the character's Unicode value in hexadecimal. This example demonstrates how one can separate out Unicode characters from a word.